4 ms·

It's not only about altitude (going to space is easy), it's also about horizontal velocity (staying in space is hard: 8 km/sec is a lot). I dont recall the exa

It's not only about altitude (going to space is easy), it's also about horizontal velocity (staying in space is hard: 8 km/sec is a lot). I dont recall the exact number, but I think that altitude is 50% of the energy, the other 50% is horizontal velocity. So building a rocket on top of the Everest would save us maybe 5% (10 km vs 100 km altitude), that's not much

adrianN 7y ago
I was thinking at least LEO, not Everest.
- NeutronStar 7y ago
  That's still only 10% less gravity, the thing is the ISS goes really fast sideways. Being high isn't enough.
- marcosdumay 7y ago
  LEO is as much a speed as it is a height.
  - adrianN 7y ago
    Obviously? When you want to manufacture things you probably don't want to fall back down right away.
    - marcosdumay 7y ago
      Well, I misunderstood your gravity wheel comment for a static position too.
- Nodraak 7y ago
  Indeed, sorry. I focused on "higher up" while the important part was "gravity well"
loeg 7y ago
OP wasn’t talking about just altitude.
petschge 7y ago
The split is actually much more uneven, and heavily on the side of kinetic energy. An orbit at 200 km altitude has an orbital velocity of 7.79 km/s. Potential energy is given by m * g * h (to good approximation, as h is much smaller than the Earth radius of 6300 km) and kinetic energy goes as 0.5 m v^2. Per unit mass (that appears identically in both energy forms) we have potential energy of g * h = 9.81 m / s^2 * 2e5 m = 1.96e6 m^2/s^2. For kinetic energy we find 0.5 * (7.79e3 m/s)^2 = 3e7 m^2/s^2. So only 6% of energy are potential energy, 94% are kinetic energy. For a relatively high orbit with 1500 km and 7.12 km/s the ratio becomes a more even 37% to 63%. If we include the extra 1.5km/s of delta-v that we loose to drag it becomes 28% to 72%. At the typical parameters of stage separation from the first stage the split is 3% to 97%. This is also why replacing the first stage by an airplane (that only gets you altitude, not that much speed) does buy you a lot less than you might think at first. We still need a rocket to go to space, even if you start 12km up.
- ta1234567890 7y ago
  Thank you for the insightful and illustrative calculations :) What would happen if we were able to make it all the way up to space, vertically, but not gain any horizontal speed?
  - Tuna-Fish 7y ago
    It would only negate 6% of the cost of going to orbit. And you'd have to pay that 94% pretty quickly to not fall down. Except that it would be better than that, because rocket nozzles work best at a specific external pressure, and getting to launch your rocket in vacuum means you can design your engines for strictly that. There are occasionally some ideas of launching rockets from tops of tall mountains, like Kilimanjaro. The advantage directly gained from being 6km closer to space is negligible, but the advantage gained from being able to use more expanded nozzles would be substantial -- albeit likely not worth having to haul your rocket up a mountain to launch it.
  - oh_sigh 7y ago
    You would just fall back to earth, gaining a lot of speed and then getting destroyed by the atmosphere. Even the force of gravity where the ISS is is pretty similar to at sea level.
drjesusphd 7y ago
Using ISS parameters, it's more like 8:1, with it taking much more energy to get to speed than altitude. "Low earth orbit is halfway to anywhere in the solar system."

adrianN 7y ago

I was thinking at least LEO, not Everest.

NeutronStar 7y ago
That's still only 10% less gravity, the thing is the ISS goes really fast sideways. Being high isn't enough.
marcosdumay 7y ago
LEO is as much a speed as it is a height.
- adrianN 7y ago
  Obviously? When you want to manufacture things you probably don't want to fall back down right away.
  - marcosdumay 7y ago
    Well, I misunderstood your gravity wheel comment for a static position too.
Nodraak 7y ago
Indeed, sorry. I focused on "higher up" while the important part was "gravity well"

loeg 7y ago

OP wasn’t talking about just altitude.

petschge 7y ago

The split is actually much more uneven, and heavily on the side of kinetic energy. An orbit at 200 km altitude has an orbital velocity of 7.79 km/s. Potential energy is given by m * g * h (to good approximation, as h is much smaller than the Earth radius of 6300 km) and kinetic energy goes as 0.5 m v^2. Per unit mass (that appears identically in both energy forms) we have potential energy of g * h = 9.81 m / s^2 * 2e5 m = 1.96e6 m^2/s^2. For kinetic energy we find 0.5 * (7.79e3 m/s)^2 = 3e7 m^2/s^2. So only 6% of energy are potential energy, 94% are kinetic energy. For a relatively high orbit with 1500 km and 7.12 km/s the ratio becomes a more even 37% to 63%. If we include the extra 1.5km/s of delta-v that we loose to drag it becomes 28% to 72%. At the typical parameters of stage separation from the first stage the split is 3% to 97%. This is also why replacing the first stage by an airplane (that only gets you altitude, not that much speed) does buy you a lot less than you might think at first. We still need a rocket to go to space, even if you start 12km up.

ta1234567890 7y ago
Thank you for the insightful and illustrative calculations :) What would happen if we were able to make it all the way up to space, vertically, but not gain any horizontal speed?
- Tuna-Fish 7y ago
  It would only negate 6% of the cost of going to orbit. And you'd have to pay that 94% pretty quickly to not fall down. Except that it would be better than that, because rocket nozzles work best at a specific external pressure, and getting to launch your rocket in vacuum means you can design your engines for strictly that. There are occasionally some ideas of launching rockets from tops of tall mountains, like Kilimanjaro. The advantage directly gained from being 6km closer to space is negligible, but the advantage gained from being able to use more expanded nozzles would be substantial -- albeit likely not worth having to haul your rocket up a mountain to launch it.
- oh_sigh 7y ago
  You would just fall back to earth, gaining a lot of speed and then getting destroyed by the atmosphere. Even the force of gravity where the ISS is is pretty similar to at sea level.

drjesusphd 7y ago

Using ISS parameters, it's more like 8:1, with it taking much more energy to get to speed than altitude. "Low earth orbit is halfway to anywhere in the solar system."